The present invention relates generally to adaptive control mechanisms such as rate control and power control for reverse link channels in CDMA networks and, more particularly, to a method of estimating load at a radio base station in a CDMA network.
CDMA networks are interference-limited systems. Since all mobile stations operate at the same frequency, internal interference generated within the network plays a critical role in determining system capacity and signal quality. The transmit power from each mobile station contributes to the reverse link load at the base stations and needs to be controlled to limit interference while maintaining desired performance objectives, e.g., bit error rate (BER), frame error rate (FER), capacity, dropped-call rate, coverage, etc. If the reverse link load becomes too high, widespread outages may occur. An outage is considered to occur when the power required to maintain minimum signal quality standards is greater than the maximum transmit power of the mobile station.
One measure of reverse link load at a radio base station is called Rise Over Thermal (RoT). RoT is generally defined as the ratio between the total received power from all mobile stations at a base station and the thermal noise. RoT is an important measure for reverse link rate control, call admission control and reverse link scheduling for high-speed packet data channels. Accurate measure of RoT has been a long standing problem. While it is relatively easy to measure the sum of the interference and background noise at the base station, it is more difficult to determine the ratio of the two, i.e. RoT, which is the measure of interest. The problem is that the base station almost always has traffic, so there is no opportunity to measure background noise. The present invention relates generally to adaptive control mechanisms such as rate control and power control for reverse link channels in CDMA networks and, more particularly, to a method of estimating load at a radio base station in a CDMA network.
The problem in determining RoT resides in determining the noise power of the background noise (including inter-cell and adjacent carrier interference) in a communication system. Two techniques have been used in the past to determine background noise power. One technique is to estimate the RoT based on estimates of the signal-to-interference-plus-noise ratio (SINR) for signals being demodulated at the base station. This technique, however, yields only a crude estimate of RoT.
A second technique for determining RoT is to silence all mobile stations in the entire network at periodic intervals and measure the noise power during the silence periods. Since no signals are received at the base station from mobile stations during the silence periods, the base station can accurately measure the background noise power. The ratio of the total received power at the base station to the background noise power yields the RoT.
The idea of measuring background noise during periodic silence periods has been proposed for systems where the reverse link frames from all mobile stations are time-aligned and where the data traffic is delay-tolerant. Systems where periodic silence periods have been proposed include IS-856 and 1xEV-DO systems. Because reverse link frames on these systems are synchronous, it is a relatively easy task to silence all mobile stations for a single frame. Also, because the systems were entirely new, the systems do not have to maintain compatibility with legacy mobile stations. Another problem with prior art solutions is that no provision has been made to account for adjacent carrier interference in multi-carrier systems.